Stuart Black, age 14, of Burnsville, N.C., for his question:

HOW DOES COLOR FILM WORK?

After exposure, color film contains images of blue, green and red areas. When the film is developed, the exposed silver salts change to metallic silver. A negative silver image forms in each layer of the film. Then a colored dye forms over each image. The silver is then bleached out of each image, leaving the dye.

In developing color film, a yellow dye covers the image made by blue light, a bluish red dye covers the image made by green light and a bluish green dye covers the image made by red light. When the negative is printed, each dye holds back light of its complementary color. The yellow dye absorbs blue light and lets red and green pass through, the bluish red absorbs green light and lets blue and red pass through while the bluish green dye absorbs red light and lets blue and green pass through.